Transcript Rings of the same size move at the same rate

Forces and Motion
Physics explains things that are
very, very large.
Physics
explains
things that
are very,
very small.
Physics explains things that are
right in front of us.
Fermilab
Force
Here are some ways of
describing forces
•
•
•
•
•
•
A push
A pull
A stretch
A squeeze
A catch
A twist
Forces can make things
•
•
•
Main Injector,•
2 miles around
Speed up
Slow down
Change direction
Change shape
Measuring Force
• Force is measured in Newton (N)
• The more the Newtons the greater the force
• One tool that measure force is a spring scale
Newton’s First Law
• Objects at rest remain at rest
• Objects in motion remain in motion
UNTIL YOU APPLY A FORCE
Objects tend to
resist a change
in motion. This
is called:
Inertia
Newton’s Second Law
F = ma
a=F
m
M= F
a
Calculate “a”
• A cart rolls down a ramp.
• The cart has a mass of 500 grams (0.5 kg).
• Using a spring scale, you measure a net
force of 2 newtons pulling the car down.
• Calculate the acceleration of the cart.
Calculate “a”
• Three people are pulling on a wagon
applying forces of 100 N,150 N, and 200 N.
• The wagon has a mass of 25 kilograms.
• Determine the acceleration and the direction
the wagon moves
Calculate force
• An airplane needs to accelerate at 5 m/sec2
to reach take-off speed before reaching the
end of the runway.
• The mass of the airplane is 5,000 kilograms.
• How much force is needed from the engine?
Calculate Force
• A tennis ball contacts the racquet for much
less than one second.
• High-speed photographs show that the
speed of the ball changes from -30 to +30
m/sec in 0.006 seconds.
• If the mass of the ball is 0.2 kg, how much
force is applied by the racquet?
Equilibium
• The condition of zero acceleration is called
equilibrium.
• In equilibrium, all forces cancel out leaving
zero net force.
• Objects that are moving at constant speed
and direction are also in equilibrium.
• A static problem usually means there is no
motion.
Equilibrium
• A woman is holding two dogs on a leash.
• If each dog pulls with a force of 80
newtons, how much force does the woman
have to exert to keep the dogs from
moving?
Quiz
• What is a force?
• 2. Can you see forces?
• 3. Can you name any effects that forces
have?
• 4. What units do we measure forces in?
• 5. What instrument do we use to measure
force?
Weight due to gravity (down)
Amount of weight keeping board
on ramp
Amount of weight making board
move down ramp
Although weight doesn’t change, the amount making the skateboard move does
Boards hit bottom at same time
Weight (force) goes up, but so
does mass (inertia).
The two cancel out, so the two
skateboards move at the same
rate.
What do we expect to see with rings of the same size ?
Rods roll faster than rings
Rings of the same size
move at the same rate
Rods roll faster than rings
Do the shapes matter?
Rods roll faster than rings
Rods roll faster than rings
Rings of the same size move at the same rate
Rods roll faster than rings
In rolling cases, mass doesn’t matter.
Shape does.
Newton’s Third Law
For every action there is an equal and
opposite reaction.
Conservation of Momentum
If m1 = m2 , then v1 = v2